1½ cups unsalted butter, cut into cubes and cool, but not cold
2 teaspoons mint extract
1 teaspoon clear imitation vanilla extract
pinch of salt
green food coloring
maraschino cherries for topping (optional)
For the cake:
Preheat the oven to 350°F. Prepare two 8" x 2" or 9" x 2" round pans by greasing and flouring; or lining with parchment, then greasing the parchment.
Mix all of the dry ingredients on slow speed to blend. Add the soft butter and mix until evenly crumbly, like fine damp sand. It may form a paste, depending on the temperature of the butter, how much it's mixed, and granulation of the sugar used.
Add the egg whites one at a time, then the whole egg, beating well after each addition to begin building the structure of the cake. Scrape down the sides and bottom of the bowl after each addition.
In a small bowl, whisk the yogurt (or milk) with the vanilla and almond extracts. Add this mixture, ⅓ at a time, to the batter. Beat 1 to 2 minutes after each addition, until fluffy. Be sure to scrape down the sides and bottom of the bowl.
Pour the batter into the prepared pans. Bake for 25 to 30 minutes for 8" or 9" rounds. Remove from the oven, remove from the pan, cool on a rack completely before frosting.
For the frosting:
Wipe the bowl of an electric mixer with paper towel and lemon juice, to remove any trace of grease. Add egg whites and sugar, and simmer over a pot of water (not boiling), whisking constantly but gently, until temperature reaches 140°F.
With whisk attachment of mixer, begin to whip until the meringue is thick, glossy, and the bottom of the bowl feels neutral to the touch (this can take up to 10 minutes or so).
Switch over to paddle attachment and, with mixer on low speed, add butter cubes, one at a time, until incorporated, and mix until it has reached a silky smooth texture (if curdles, keep mixing and it will come back to smooth). Add extracts and salt, and beat until well combined.
Put about 1-1½ cups of frosting into a piping bag with a star tip. This will be used as your "whipped cream".
Add green food coloring to the remaining frosting and mix well. Frost with green frosting between layers and on top and sides of cake. Then pipe the white frosting whipped cream dollops in the center and perimeter of the cake. Top each dollop with a maraschino cherry.
Recipe by at http://www.theculinaryenthusiast.com/2015/03/11/shamrock-shake-celebration-cake/